1Field of the Invention
The invention relates to a method and a device for assessing the quality of service (QoS) on transmission channels in a digital transmission system, in which, for a channel coding, a turbo coding is carried out in a turbo coder at the transmitter end and a turbo decoding is carried out in a turbo decoder with soft-decision output signals at the receiver end.
The use of turbo codes for digital transmission systems is investigated in the article xe2x80x9cComparison of Turbo-Code Decoders Applied to Short Frame Transmission Systemsxe2x80x9d, by P. Jung, IEEE Journal on Selected Areas in Communications, Volume 14 (1996) pages 530-537, with both coders and decoders being investigated for the turbo codes in the transmission path. Decoding of the turbo codes is based on the use of soft-input/soft-output decoders, which can be produced using either MAP (maximum a posteriori) symbol estimators or MAP sequence estimators, for example an estimator using an a-priori soft-output Viterbi algorithm (APRI-SOVA). This publication describes four different decoder configurations and their capabilities to process specific error rates. Furthermore, the performance of these decoders is investigated for different applications. It has been found that the turbo codes and their iterative decoding are an effective measure against packet errors.
In the article xe2x80x9cTurbo Codes for PCS Applicationsxe2x80x9d, ICC ""95, Seattle, Wash., Jun. 18-22, 1995, D. Divsalar and F. Pollara propose turbo codes to achieve an error correction virtually as far as the so-called Shannon limit. Relatively simple component codes and large interleavers are intended to be used for this purpose. In this publication, the turbo codes are produced in a coder using multiple codes, and are decoded in a suitable decoder. The turbo codes were introduced by Berrou et al. 1993 (see C. Berrou, A. Glavieux and P. Thitimayshima, xe2x80x9cNear Shannon limit area correction coding: Turbo codesxe2x80x9d, Proc. 1993 IEE International conference on communications, pages 1064-1070). On the one hand, this method allows very good error correction to be achieved.
So-called turbo equalization is known from the article xe2x80x9cIterative Correction of Intersymbol Interference: Turbo-Equalizationxe2x80x9d, by Catherine Douillard et al., ETT European Transactions on Telecommunications, Vol. 6, No. 5, September-October 1995. The use of turbo equalization is intended to overcome the disadvantageous effects of intersymbol interference in digital transmission systems which are protected by convolution codes. The receiver makes two successive soft-output decisions, which are made in an iterative process by a symbol detector and a channel decoder. Each iteration makes use of extrinsic information from the detector and the decoder for the next iteration, as with turbo decoding. It was found that intersymbol interference effects in multipath channels can be overcome by turbo equalization.
M. Nahan et al. disclose in their article xe2x80x9cNew Results on the application of antenna diversity and turbo-codes in a JD-CDMA mobile radio systemxe2x80x9d, 5th IEEE International Sysposium on Personal, Indoor and Mobile Radio Communications (PIMRC ""94), and ICCC Regional Meeting on Wireless Computer Networks/WCN), The Hague, The Netherlands, Vol. 2, Sep. 18-23, 1994, pages 524-528 how assessment of the system behavior (system performance) can be achieved in a JD (joint detection) -CDMA (code division multiple access) mobile radio system by long-term averaging of the variances of the soft-decision output signals ("sgr"LLR2) from a turbo decoder.
Future transmission systems, for example the European UMTS (Universal Mobile Telecommunications System), require the support of a large number of co-existing carrier services with carrier data rates of up to 2 Mbit/s in a flexible manner, with the best-possible spectral efficiency being desirable. An MA (Multiple Access) scheme has been developed in the ACTS (Advanced Communications Technologies and Services) project AC090 FRAMES (Future Radio Wideband Multiple Access Systems), which is called FRAMES Multiple Access (FMA) and satisfies the UMTS requirements. As a third-generation transmission system, which covers a wide range of application areas, carrier services and widely differing scenarios, FMA must comply with present and future developments of UMTS radio interface standards. FMA includes two operating modes, namely WB-TDMA (Wideband Time Division Multiple Access) with and without spreading and compatibility with GSM (Global System for Mobile Communications) and WBCDMA (Wideband Code Division Multiple Access). Although, essentially, a system based on FMA is considered here, it is also possible to include other transmission systems using multiple access methods, for example FDMA (Frequency Division Multiple Access), MC-CDMA (Multicarrier-CDMA) or combinations of the transmission systems.
With regard to the high performance of turbo codes, it is desirable to use these in digital transmission systems. The complex requirements, for example for FMA, mean, however, that it is necessary when using such turbo codes to ensure that the data transmission is not overloaded by transmission of error correction codes.
It is accordingly an object of the invention to provide a method and a device for assessing the quality of service (QoS) on transmission channels in a digital transmission system which overcome the above-mentioned disadvantages of the heretofore-known methods and devices of this general type and which do not require any additional complexity or outlay.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for assessing a quality of service on transmission channels in a digital transmission system, which includes the steps of:
performing a turbo coding in a turbo coder for a channel coding at a transmitter end;
performing a turbo decoding in a turbo decoder with soft-decision output signals at a receiver end; and
determining a quality of service from instantaneous values of variances of the soft-decision output signals from the turbo decoder.
In other words, according to the invention, the method mentioned above is characterized in that the quality of service is determined from the instantaneous values of the variances of the soft-decision output signals from the turbo decoder.
In the following, the term quality of service is used as follows. Specific QoS criteria (QoS=Quality of Service) apply to various services, and the definitions of the QoS criteria for various carrier services have been worked out in the course of FRAMES. One important component of a QoS criterion is the carrier data rate R. The QoS criterion also includes a maximum permissible error rate PbG or a packet loss rate P1G in conjunction with a maximum failure probability PoutG. In the case of line-switching services, the probability P{Pb greater than PbG} of the instantaneous bit error rate Pb exceeding PbG must not be greater than PoutG, that is to say
Pr{Pb greater than PbG} less than PoutG
For voice transmission, PbG is equal to 10xe2x88x923 and PoutG is equal to 0.05. A similar condition for the instantaneous packet loss rate P1 applies to packet services:
Pr{P1 greater than P1G} less than PoutG
Apart from the criteria relating to Pr, there are also other conditions relating to the QoS criterion. However, the QoS parameters PbG, P1G and PoutG will mainly be considered here, which relate directly to the choice of the error correction code (ECC). For the ECC, the coding rate Rc is essentially governed by the multiple access method, the modulation and the packet parameters. In other words, the coding rate Rc is directly related to the question as to whether a QoS criterion is or is not satisfied for a specific service.
It can be seen that determination of the quality of service in conjunction with the QoS criteria is important, wherein this determination is possible by using the method according to the invention without having to accept any additional complexity or outlay, since the variances at the receiver end can be determined without any additional information.
In a method in which a soft-input/soft-output symbol or sequence estimator is used at the receiver end, it is advantageous for the quality of service to be determined from the variances "sgr"2 of the soft-decision output signals from the turbo decoder, with the bit error rate advantageously being calculated from the variances "sgr"2, as a measure of the quality of service.
In a method in which an MAP symbol estimator or an MAP sequence estimator is used at the receiver end, it is advantageous for the quality of service to be determined from the variances "sgr"2LLR (LLR=log-likelihood ratios) of the soft-decision output signals from the turbo decoder.
In a method in which a Viterbi algorithm is used for a sequence estimation at the receiver end, it is advantageous for the quality of service to be determined from the variances "sgr"2VIT of the soft-decision output signals from the turbo decoder.
According to another mode of the invention, a Berrou""s puncturing is used for puncturing only non-systematic information.
In accordance with a further mode, a UKL puncturing is used for puncturing both, systematic information and non-systematic information.
In accordance with yet another mode of the invention, the number of decoding iterations is optimized by taking into account a time delay associated with the decoding iterations and an improvement of the decoding result.
Since the method according to the invention can be used not only with MAP estimators but also for an estimation or assessment using a Viterbi algorithm, there is virtually no limitation with regard to the most important methods for sequence and symbol estimation. This is true even though this statement is specified only in conjunction with an MAP symbol estimator in the following specific description.
Furthermore, it is advantageous in the method according to the invention for an RCPTC (Rate Compatible Punctured Turbo Code) to be used as the turbo code. As can be seen from the detailed description, such a code is particularly flexible and suitable for the purposes of the invention. The RCPTC allows the transition from defined ECC strategies to flexible ECC strategies, with the latter being matched to requirements that vary with time, so that flexible linklayer control and medium access control is possible.
With the objects of the invention in view there is also provided, a device for assessing a quality of service on transmission channels in a digital transmission system, including:
a transmitter-side turbo coder;
a receiver-side turbo decoder, operatively connected to the turbo coder, for outputting soft-decision output signals;
a computation device connected to the turbo decoder for calculating a quality of service from instantaneous values of variances of the soft-decision output signals from the turbo decoder.
In other words, a device for assessing the quality of service on transmission channels in a digital transmission system having a turbo coder at the transmitter end and having a turbo decoder at the receiver end, which emits soft-decision output signals, is characterized by a computation device which calculates the quality of service from the instantaneous values of the variances of the soft-decision output signals from the turbo decoder in order to carry out the method described above.
In accordance with an advantageous feature of the invention, the turbo coder has two parallel-connected RSC coders, an interleaver which is connected upstream of one of the RSC coders, and a puncturing/multiplexer device, to which the systematic sequences and the coded sequences from the RSC coders are supplied, and the turbo decoder has two RSC decoders, a turbo-code interleaver between one output of the first RSC decoder and one input of the second RSC decoder, as well as a turbo-code deinterleaver between one output of the second RSC decoder and one input of the first RSC coder. This configuration of the turbo coder and of the turbo decoder represents a simple solution for these two components, wherein the solution is adequate for most applications and wherein the RCPTC that is produced satisfies all the requirements.
In accordance with another feature of the invention, the puncturing/multiplexer device carries out a puncturing using the so-called Berrou""s puncturing process, in which only the non-systematic information is punctured. This type of puncturing is advantageous if the signal-to-noise ratio values are relatively low.
A further advantageous embodiment of the device according to the invention is characterized in that the puncturing/multiplexer device carries out a puncturing using the UKL (UKL=University Kaiserslautern) puncturing process, in which both the systematic information and the non-systematic information are punctured. This type of puncturing is advantageous for higher signal-to-noise ratios, and thus for bit error rates of  less than 10xe2x88x924.
In accordance with a further feature of the invention, the number of decoding iterations in the turbo coder is optimized taking into account the time delay, associated with this, and the improvement of the decoding result.
In accordance with an advantageous mode of the invention, the so-called Berrou""s puncturing is used for puncturing, in which only the non-systematic information is punctured.
In accordance with another advantageous mode of the invention, a so-called UKL puncturing is used for puncturing, in which both, systematic information and non-systematic information are punctured.
In accordance with yet another advantageous mode of the invention, the number of decoding iterations is optimized taking into account the time delay, associated with this, and the improvement of the decoding result.
The two above-mentioned puncturing methods and the optimization of the number of decoding iterations are advantageously used to allow the assessment of the quality of service on the transmission channels to be carried out with greater confidence.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for assessing the service quality of transmission channels in a digital transmission system it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.